Colour-controllable lamps typically include three light sources, respectively producing red (R), green (G) and blue (B) outputs. By controlling the intensity of each of the three light sources, a user may control of both the perceived colour, or chromaticity, and the luminance, or intensity, of the lamp.
The perceived colour, or chromaticity, may be represented by two colour coordinates x and y, according to the CIE 1931 standard. This standard plots, on a two-dimensional chart, the perceived colour of light: FIG. 1 shows the chart in block form. Around the perimeter of the chart is shown the spectrum of fundamental frequencies ranging from red (R), through orange (O), yellow (Y), green (G), blue (B), Indigo (I) and violet (V). The interior of the chart demonstrates various mixtures of the colours, with the central area corresponding to white light (W). Also shown on the figure is the black body radiation curve, corresponding to the colour of radiation emitted by a black body, which follows a path from the right to the left with increasing temperature.
It will be appreciated that a user has 3 degrees of freedom in controlling the lamp—that is to say the magnitude of the each of the red, green and blue channels. Two of these degrees of freedom control the chromaticity of the output, and the third degree controls the intensity. In the case of, for instance, 12-bit digital control where each of R, G and B can be assigned values between 0 and 4095, and ignoring the variation of perceived intensity with frequency, the sum R+G+B is indicative of the luminance, and the ratios B/R and G/R are indicative of chromaticity. Of course any other two pairs of ratios may be used; the third ratio will be determined from the two pairs of ratios and the sum.
In an ideal situation, the three light sources are “perfect” in the sense that they produce respectively monochromatic R, G and B light, which has a fixed chromaticity—that is to say it has fixed X and Y, colour-coordinates, independent of operating conditions such as intensity or operating temperature.
In practice, LED light sources produce light of which the dominant frequency and width of the frequency spectrum vary with both operating temperature and intensity. Thus, correction factors have to be applied to the user inputs when controlling a RGB colour controllable LED lamp.
Recently there has been a trend towards adding a fourth, white (W), LED, to the three colour LEDs. White LEDs are generally fundamentally different to coloured monochromatic LEDs, in fact in a white LED the light output is not produced directly from an electronic transition within the device—typically from a p-n junction; rather the LED includes a phosphor, which convert a fraction of the blue light generated by the p-n junction to visible yellow light, which together generate visible white light; nonetheless the resulting white light output from a white LED also varies with operating temperature and intensity.
Control methods are known which include correction for the variation of LED output for three colour RGB LED lamps, with operating temperature. For four colour RGBW lamps, such corrections may be far more complex.